Diaphragm for manganese electrowinning



Unite S a es, a e Qfii DIAPHRAGM FOR MANGANESE ELECTROWINNIN G Robert H.Cromwell, East Orange, N. 1., and William A.

Parsons, Knoxville, Tenn., assignors, by mesne assignments, to FooteMineral Company, Philadelphia, Pa., a corporation of Pennsylvania NoDrawing. Application October 21, 1954, Serial No. 463,826

5 Claims; ((31.204-105) direct electric current ispassed between ananode and a cathode through a purified manganese sulfate-ammoniumsulfate electrolyte. The electrolyte adjacent the anode (the anolyte) isof markedly different composition than the'electro'lyte adjacent thecathode (the catholyte). Both electrolytes generally contain from 120 to200 grams perliter of ammonium sulfate; but the catholyte is distinctlyalkaline at a pH of about 8 to 9 and contains from 30 to 40 grams perliterof manganese (as manganous sulfate), whereas the anolyte isstrongly acid with a pH in the range from 0.8 to 1.4 and contains from12 to 16 grams per liter of manganese.

g A diaphragm must be employed in the electrolytic cell to separate theanolyte from the catholyte. The diaphragm must provide for effectivephysical separation of the anolyte from the catholyte so that no excessdiffusion of one into the other takes place. However, the processinvolves feeding. strong electrolyte high in manganese .into thecatholyte compartment of the cell, where it becomes depleted inmanganese as the metal is deposited on the cathode, and correspondinglywithdrawing anolyte from the anolyte compartment, where it becomesenriched in sulfuric acid in proportion to the amount of manganesedeposited. Consequently the diaphragm must be sufficiently permeable tothe electrolyte so as to permit it to fl0W from the catholytecompartment into the anolyte compartment at essentially the same rate asfresh electrolyte is fed into the catholyte. Moreover the diaphragm mustpermit the flow of the electric current through it without introducing avery high electrical resistance into the current path. The diaphragmmust also be capable of withstanding the severe chemical environment towhich it is subjected in normal use. It must withstand the attack of analkaline ammonium hydroxide catholyte on one side while it issimultaneously exposed on the other side to a strongly acidic sulfuricacid anolyte. Moreover, some of the manganese salts present in theelectrolyte are unavoidably converted to manganese cit ids and topermanganates, and these compounds subject the diaphragm to severeoxidizing influences.

Prior to the present invention, and since the beginning of thecommercial art of manganese electrowinning, a heavy cotton canvas orduck has been the only satisfactory material of which tomake thediaphragm. The most satisfactory cotton fabric diaphragm material hasbeen heavy duck, weighing about 21 ounces per square yard, woven of3-ply strands with 29 filling strands and 36 warp strands per squareinch. The life of such a diaphragm is only about six to eight weeks innormal service. At the end of such period of time, the pores of thefabric have become so plugged with a deposit of salts from theelectrolyte that the diaphragm is no longer adequately permeable.Moreover, it has become so weakened physically by the chemical action ofthe electrolyte that it is usually impractical, and in many cases evenimpossible, to clean it. The cost of labor and materials for replacingthese heavy cotton duck diaphragms is of course very high, be ing, forexample, upwards of $25,000 per year in a plant operating, say, seventycells with twenty diaphragm chambers per cell.

It has long been recognized that important economies could be achievedin the electrolytic manganese process were it possible to provide adiaphragm which would not need replacement at the end of each cell cycleof six to eight weeks or thereabouts; and much effort has been devotedto a search for a satisfactory diaphragm to replace cotton duck. Earlyattempts were made to make diaphragms of wool, which is highly acidresistant, but the wool is dissolved by the alkaline catholyte and so isless satisfactory than cotton. The various rayons, including theviscose, acetate and cuprammonium products, proved unsatisfactorybecause of low wet strength and inability to resist gradual stretchingwith objectionable increase in permeability. The acetate rayons,moreover, were attacked by prolonged exposure to the acid anolyte, andall of the rayons are weakened by the chemical attack of theelectrolyte. Nylon proved entirely unsatisfactory because of itssolubility in the acid anolyte. A diaphragm of woven glass fiber clothseemed promising, but it proved impossible to weave a cloth of glassfibers having the requisite tightness of weave and corresponding low butdefinite permeability. Moreover, the glass fibers were slowly attackedby the alkaline catholyte, leading to the contamination of theelectrolyte with dissolved silica which has been found to be a mostobjectionable impurity in the electrolyte solution. Vinyona fabric wovenof fibers of the copolymer of vinyl chloride and vinyl acetate-seemed tobequite resistant chemically to the electrolyte, but diaphragms made ofit stretched so much under their own weight after a period of immersionin the electrolyte and their permeability so increased that they ceasedto provide effective separation of the catholyte from the anolyte.Micropore rubber, a sheeted foamed rubber having numerous minute poresextending through it was tried but was found to stretch objectionablyunless supported on a metal backing. The introduction of a metal-backedproduct into the electrolyte was most ob jectionable, but might havebeen tolerated had it not been for other defects of the rubber. One suchdefect was a lack of uniformity in its permeability. Another was that itbecomes oxidized and embrittled by the man ganese dioxide andpermanganates which form in the electrolyte, and the sulfur,accelerators, and other substances present in the rubber are oxidized tothionates and other objectionable contaminants in the electrolyte. Forthese reasons, micropore rubber and other rubberized diaphragm materialsare impractical. Asbestos fiber diaphragms proved to be mechanicallyweak and heavy, and additionally were subject to the same disadvantageas glass fiber diaphragms -silicon dissolves in the alkaline catholyteand leads to objectionable contamination of the to give the requisitepermeability without so much non Patented Oct. 22, 1957 uniformity as toresult in excessive permeability over some areas of the diaphragm sheet.

Thus, although many efforts have been made and numerous materials hadbeen tried to find a betterdiaphragm, cotton duck remained, for manyyears,th'e most practical and only commercially, satisfactory diaphragmmaterial available to the art. The present invention provides the firstacceptable substitute for a cotton diaphragm'that has been developed,and the first which has made it possi ble to operate more economicallythan withcotton dia phragms. Strangely enough, consideringthe largenumber of materials that have proved unsatisfactory, we have now foundthat eminently satisfactory diaphragms can be made from fibers of eitherlinear homopolymers of acrylonitrile or polytetrafluoroethylene.

The suitability of diaphragms made of fabrics woven from fibers of theseparticular synthetic resinous materials resides not simply in the factthat such fibers are resistant to chemical attack by the electrolytes,or even simply in the fact that such fabrics possess high wet strengthand other good physical properties. These properties are of courseimportant, but a major contributing factor to the success of theinvention lies in the fact that these fibers lend themselves to theconstruction of a closely woven fabric which possesses, and whichretains over prolonged periods of time and through repeated cycles ofuse in the electrolytic cells, a satisfactory permeability to theelectrolytes-a permeability which is low enough to prevent objectionableintermixing of anolyte and catholyte, and yet which is high enough toallow the required seepage of catholyte into the anolyte compartment aselectrodeposition of manganese proceeds. The invention, therefore, inaddition to being characterized by the use of a diaphragm made of afabric woven of one of the above-mentioned synthetic fibers, is furthercharacterized by the fact that such fabric is closely woven and has amaximum permeability measured by the flow of catholyte therethrough intothe anolyte of not more than about 25 cubic centimeters per square footper minute under a hydrostatic head of catholyte with respect to anolyteof /2 inch, and by the ability of such fabric to resist physicaldeformation and any significant change (especially any increase) in itspermeability characteristics during or in consequence of prolongedexposure to the electrolyte.

The minimum permissible permeability of the diaphragm material isdetermined primarily by economic considerations. Theoretically, thediaphragm can be used if it possesses any degree of permeability. As apractical matter, however, if the permeability is too low, the seepageof catholyte through it will be too slow to'meet the demands of aneconomic rate of electrodeposition. Also, if the permeability is verysmall, the diaphragm will have the effect of imposing a correspondinglyhigh resistance in the current path between anode and cathode, and as aresult the cell voltage drop due to the presence of the diaphragm willbe so high as to result in uneconomically high power consumption.Accordingly, the permeability of the diaphragm, in terms of the flow ofcatholyte therethrough into the anolyte, should be at least about 20cubic centimeters per square foot per minute under a hydrostatic head ofcatholyte with respect to anolyte of 4 inches; and it should be greatenough so that the overall cell voltage from anode to cathode is notmore than 0.3 volt greater than in a comparable cell equipped with acotton duck diaphragm weighing substantially 21 ounces per square yard.

In normal commercial operations, we prefer to employ a diaphragm havinga permeability such that the flow of catholyte therethrough is about 22cubic centimeters per square foot per minute under a hydrostatic head ofcatholyte with respect to anolytein the range from A to 1 /2 inches.

The electrolyte employed in the electrolytic manganese process generallyis saturated, or nearly so, with calcium sulfate, and this salt tends tocrystallize in the pores of the diaphragm fabric, particularly on theanode-facing side thereof where the electrolyte is strongly acidic. Theaccumulation of calcium sulfate (and other compounds as well) on and inthe diaphragm material reduces its permeability. Consequently, after adiaphragm has been in service for a substantial period of time,generally from about six to eight weeks, the permeability of thediaphragm has become reduced to the point Where it no longer functionssatisfactorily. As stated above, cleaning and reusing the cottondiaphragms heretofore employed, after they have become thus plugged withsalts, is quite impractical. It has therefore been customary to removesuch diaphragms from the cell and replace them with new diaphragms.However, cleaning and reusing diaphragms of the invention, woven fromfabrics of the above-specified synthetic fibers, is not only possiblebut is necessary in order to secure the full benefits of the invention.The fabrics used for making diaphragms in accordance with the inventionare more costly than cotton, and it is only by making repeated use ofeach diaphragm, over a considerable number of cell cycles, that theimportant economies which the invention contemplates are realized.

In accordance with the invention, therefore, the new diaphragms arecleaned to remove the accumulation of salts after they have been in usefor six to eight weeks, and they are then reused. This cycle of use forsix to eight weeks followed by cleaning preparatory to further use mayin accordance with the invention be repeated many times. Hence, althoughthe initial cost of diaphragms made in accordance with the invention maybe substantially higher than for cotton diaphragms, the ultimate cost ismarkedly less. Indeed, on the basis of our experience to date withoperations carried out in accordance with the invention, it isconservative to state that approximately four-fifths of the cost fordiaphragms can be saved by using diaphragms of the character hereinspecified and repeatedly cleaning them, as compared with using cottonduck diaphragms and replacing them after each cycle of use, as hasheretofore been customary. In the case of the plant hereinbeforementioned, in which the cost for cotton diaphragms in accordance withheretofore customary practice would be approximately $25,000 per year,the saving effected in accordance with the invention may amount tofour-fifths of this total, or $20,000 per year.

Various procedures may be employed to clean the diaphragms in carryingout the method of this invention.

In general, however, the cleaning operation involves the step ofsupporting the diaphragm in a cleaning tank and maintaining fresh waterunder slight pressure in contact with the cathode-facing surface thereofuntil the calcium sulfate deposits are loosened. The water pressurerequired is low, being only that necessary to keep the anode-facingsurface of the diaphragm wet (preferably dripping wet) by seepage ofwater through the diaphragm. The cleaning method can be carried out bysuitably supporting the diaphragm assemblies and filling them withwater, or immersing them in a tank of water, whichever may be requiredto apply water to the cathode-facing surface. The hydrostatic head ofthe water in the diaphragm or in the cleaning tank, acting on thediaphragm, provides all the pressure that is necessary for the requisiteseepage of water through the diaphragm. In the course of a few hours toa day or so, the fresh water seeping through the diaphragm dissolvesenough of the calcium sulfate crystals where they penetrate the meshesof the fabric so that the whole deposit sloughs off or can easily bewashed off.

The following examples illustrate the manner in which the invention iscarried out:

Example I A diaphragm was prepared using a heavy'tightly woven.

Oi-Ion" and composed essentially of polyacrylonitrile, a linearhomopolymer with head-to-tail linkage of acrylonitrile, havingsubstantially the formula [-CH2CH(CN)-]n, in which-n is an integergenerally approximating 1500 but which, for a small proportion of thepolymer, has other values extending as low as 350 and as high as 2200,the average molecular weight of the polymer being about 80,000 and theindividual polymeric constituents having molecular weights in the rangefrom about 19,000 to about 110,000. The fabric was woven from 5-ply 80filament 200 denier filling strands and -ply 80 filament 200 denierwarpstrands, using 75 warp strands and 23 filling strands per linearinch. The fabric weighed about 18.5 ounces per square yard. Thediaphragm was mounted in a conventional electrolytic manganese cell,where it was exposed on one side to an acidic anolyte having a pHgenerally in the range of 0.9 to 1.2 and containing generally from 125to 140 grams per liter of ammonium sulfate and about grams per liter ofmanganese (as manganous sulfate), and on the other side to an alkalinecatholyte having a pH of about 8 to 9 and containing approximately thesame amount of ammonium sulfate as the anolyte and about 35 grams perliter of manganese. The tempera ture of both anolyte and catholyte wasin general maintained in the range from 35 to 45 C. Electrolyticmanganese was deposited on the cathode immersed in the catholyte, bypassing a direct electric current between the anode and cathode throughthe electrolytes and the diaphragm, over a period of about eight weeks.At the end of such period, a substantial crust of calcium sulfatecrystals had collected on the anode-facing side of the diaphragm. Thediaphragm assembly was removed from the cell, and was filled with freshtap water. The fresh water seeped slowly through the diaphragm fabric tothe outer surface thereof, which in service faced the anode, and onwhich the crust of calcium sulfate crystals had collected. After aperiod of about twenty-four hours, the crust of calcium sulfate crystalshad become so loosened that it could easily be removed by hosing theouter surface of the diaphragm assembly with a stream of water. Afterfurther cleaning to remove manganese dioxide and other compounds thatalso had collected on it, the diaphragm assembly was returned to theelectrolytic cell and was reused in the manner described above. Thediaphragm showed no sign of deterioration, other than that incident tomechanical wear, over a period of many such cycles of use and cleaning.

Fabrics of fibers composed of Teflon, a long-chain polymer oftetrafluoroethylene, also may be used in accordance with the invention.The molecule of this polymer consists of a long chain of relativelysimple units having the formula CF2. Teflon polymers are available infiber form and can be woven into fabrics of the character describedabove, and having the properties required of diaphragms for use inaccordance with this invention.

We claim:

1. The method of electrowinning manganese which comprises passing anelectric current from an anode through an anolyte comprising an acidicmanganous sulfate-ammonium sulfate solution, and through a catholytecomprising an alkaline manganous sulfate-ammonium sulfate solution to acathode, characterized in that the anolyte and catholyte are separatedby a closely woven permeable fabric diaphragm having a maximumpermeability measured by the flow of catholyte therethrough into theanolyte of not more than about cubic centimeters per square foot perminute under a hydrostatic head of catholyte with respect to anolyte of/2 inch, said diaphragm being capable of resisting physical deformationand change in its permeability characteristic during prolonged exposureto the electrolytes, and being woven of fibers of a linear homopolymerof a compound from the group cons'isting'o'f acrylonitrile andpolytetrafluoroethylene. J

2. The method according to claim 1, in which calcium is present in theanolyte, characterized in that the diaphragm is periodically withdrawnfrom the electrolytic operation, deposits 'of calcium sulfate areremoved from the anode-facing surfaces thereof by supporting saiddiaphragm in a cleaning tank and maintaining fresh water under slightpressure in contact with the cathode-facing surface thereof until saiddeposits are loosened, the water pressure on the cathode-facing surfacebeing only sufficient to keep the anode-facing surface of the diaphragmwet by seepage of water through the diaphragm, and the .cleaneddiaphragm is returned to the electrolytic operation for further usetherein.

3. The method of electrowinning manganese which comprises passing anelectric current from an anode through an anolyte comprising an acidicmanganous sulfate-ammonium sulfate solution and through a catholytecomprising an alkaline manganous sulfate-ammonium sulfate solution to acathode, said anolyte and said catholyte being substantially saturatedwith dissolved calcium sulfate, the anolyte and catholyte beingseparated by a closely woven permeable fabric diaphragm initially havinga permeability, as measured by the flow of catholyte therethrough intothe anolyte, in the range from a maximum of about 25 cubic centimetersper square foot per minute under a hydrostatic head of catholyte withrespect to anolyte of /2 inch to a minimum of about 20 cubic centimetersper square foot per minute under a hydrostatic head of catholyte withrespect to anolyte of 4 inches, said diaphragm being capable ofresisting physical deformation and change in its permeabilitycharacteristics during prolonged exposure to the anolyte and catholyteand being woven of fibers of a linear homopolymer of a compound from thegroup consisting of acrylonitrile and polytetrafluoroethylene,continuously adding electrolyte high in dissolved manganese to thecatholyte and continuously withdrawing anolyte, continuing theelectrolytic operation until a substantial crust of precipitated calciumsulfate has collected on a face of the diaphragm, withdrawing thediaphragm from the electrolytic operation, introducing fresh water underslight pressure into contact with the face of said diaphragm oppositethat on which said calcium sulfate deposit hasmostly collected, thewater pressure being only sufficient to cause sutficient seepage ofwater through the diaphragm to keep dripping wet the surface on whichthe calcium sulfate deposit has mainly formed, maintaining the freshwater in contact with the diaphragm until the calcium sulfate deposithas loosened, removing the loosened deposit from the diaphragm, andreturning the cleaned diaphragm to the electrolytic operation for reuse.

4. In a process for electrowinning manganese, in which an electriccurrent is passed from an anode through an acidic manganese-bearinganolyte and an alkaline manganese-bearing catholyte to a cathode and inwhich the anolyte and catholyte are separated by a permeable diaphragm,the improvement which comprises employing as the diaphragm material aclosely woven permeable fabric capable of resisting deformation withconsequent change in permeability over a prolonged period of contactwith said electrolytes, said diaphragm being woven of fibers consistingessentially of linear homopolymers of acrylonitrile having an averagemolecular weight of about 80,000, with individual polymeric constituentshaving molecular weights in the range from 19,000 to 110,000, saiddiaphragm having a permeability measured by the flow of catholytetherethrough into the anolyte of about 22 cubic centimeters per squarefoot per minute under a hydrostatic head of catholyte with respect toanolyte in the range from to 1% inches.

5. In a process for electrowinning manganese, in which an electriccurrent is passed from an anode through an acidic manganese-bearinganolyte and an alkaline manganese-bearing catholyte to a cathode and inwhich the an- 8 References Cited in the file of this patent UNITEDSTATES'PATENTS 1,120,629 Salisbury DEC. 8, 1914 FOREIGN I PATENTS GreatBritain Q Mar. 5, 1952 667,528 OTHER REFERENCES Quig: Rayon andSynthetic Textiles, vol. 30, No. 4,

10 April, 1949, page 92.

Jacob's et a1.: Meta1 Industry, Dec. 8, 1944, pages 358, 359;

1. THE METHOD OF ELECTROWINNING MANGANESE WHICH COMPRISES PASSING ANELECTRIC CURRENT FROM AN ANODE THROUGH AN ANOLYTE COMPRISING AN ACIDICMANGANOUS SULFATE-AMMONIUM SULFATE SOLUTION, AND THROUGH A CATHOLYTECOMPRISING AN ALKALINE MANGANOUS SULFATE-AMMONIUN SULFATE SOLUTION TO ACATHODE, CHARACTERIZED IN THAT THE ANOLYTE AND CATHOLYTE ARE SEPARATEDBY A CLOSELY WOVEN PERMEABLE FABIC DIAPHRAGM HAVING AN MAXIMUMPERMEABILITY MEASURED BY THE FLOW OF CATHOLYTE THERETHROUGH INTO THEANOLYTE OF NOT MORE THAN ABOUT 25 CUBIC CENTIMETERS PER SQUARE FOOT PERMINUTE UNDER A HYDROSTATIC HEAD OF CATHOLYTE WITH RESPECT TO ANOLYTE OF1/2 INCH, SAID DIAPHRAGM BEING CAPABLE OF RESISTING PHYSICAL DEFORMATIONAND CHANGE IN ITS PERMEABILITY CHARACTERISTIC DURING PROLONGED EXPOSURETO THE ELECTROLYTES, AND BEING WOVEN OF FIBERS OF A LINEAR HOMOPOLMER OFA COMPOUND FROM THE GROUPS CONSISTING OF ACRYLONITRILE ANDPOLYTETRAFLUOROETHYLENE.